Hjorth, G
(WORLD SCIENTIFIC PUBL CO PTE LTD, 2012-06-01)
There are continuum many ≤B-incomparable equivalence relations induced by a free, Borel action of a countable non-abelian free group — and hence, there are 2α0 many treeable countable Borel equivalence relations which are incomparable in the ordering of Borel reducibility.